Piston Engines - Cooling — Page 56, Lesson 79

Right, let's get into the cooling systems for piston engines. This is a new topic, so we're starting fresh with how we manage the enormous heat these engines generate. We have two fundamental approaches: liquid cooling and air cooling. The choice between them, and the cylinder arrangement we saw earlier, all ties back to the power required and the type of cooling system used. Let's start with the liquid cooling system. The principle is simple: we pump a coolant through the engine to absorb heat, then dump that heat into the air somewhere else. The coolant here is a specific mixture — Water and Glycol, and the glycol is the anti-freeze. It's not just water, because we need to prevent freezing in cold conditions. This mixture is pumped through passages that are built directly into the cylinders and the cylinder heads. That's where the heat is generated, so that's where we absorb it. Now, the hot liquid doesn't just stay in the engine. It's passed through an Air-cooled Radiator. Here's a clever detail: that radiator is mounted in the slipstream of the propeller. That's a deliberate design choice. It ensures there is an airflow through the radiator even when the aircraft is stationary on the ground. If the radiator were only cooled by forward speed, you'd overheat the moment you stopped. The propeller slipstream guarantees cooling airflow at all times. The circulation is driven by an engine driven Pump — so the pump is mechanically powered by the engine itself. The temperature of this whole system is controlled by a Thermostat, which regulates the flow to keep the engine at its optimal temperature. The liquid is stored in a reservoir called a Header Tank. So let's trace the complete circuit: pipes carry the liquid from the header tank to the engine, then from the engine to the radiator, and then back to the header tank. The air flowing through the radiator dissipates the heat from the coolant to the air. That's the complete loop — a closed cycle that continuously moves heat from the cylinders out to the atmosphere. Now let's contrast that with the air-cooled engine. Here, there's no liquid at all. The engine uses the cooling air from the Propeller Slipstream and the Aircraft's Forward Speed to transfer the heat generated in the engine directly to the air. The heat goes straight from the metal to the passing air. But there's a problem with this direct approach: you can't just leave the engine exposed. So the engine is Cowled — enclosed in a cowling. This serves two purposes. First, it reduces drag. Second, it controls the flow of air around the engine to ensure equal cooling. That's a critical point — without control, the front of the engine would get blasted with air and cool excessively, while the rear would overheat. The cowling manages that distribution. And on some aircraft, you can actually alter the rate of that airflow. This is done with a variable Cowl Flap or Gills located at the rear of the engine cowling. By opening or closing these, the pilot controls how much air flows through the cowling and over the engine. So to summarise the key contrast: liquid cooling uses a water-glycol mixture pumped through the engine to a radiator in the propeller slipstream, with a header tank, pump, and thermostat managing the loop. Air cooling uses the slipstream and forward speed directly over the engine, with a cowling and adjustable cowl flaps or gills to control and equalise that airflow.